Composition and process for inhibiting corrosion of zinc surfaces

ABSTRACT

A PROCESS FOR PROVIDING A CORROSION RESISTANT PASSIVATING FILM ON A ZINC SURFACE, WHEREIN THE SURFACE IS TREATED WITH AN AQUEOUS SOLUTION OF A CHROMIC ACID WITH PART OF THE HEXAVALENT CHROMIUM REDUCED TO OR REPLACED BY TRIVALENT CHROMIUM, WITH THE PH OF THE SOLUTION MAINTAINED IN THE RANGE OF 2.8-3.8 BY THE ADDITION OF A PH CONTROL AGENT.

United States Patent O Int. (:1. but 7/26 U.S. Cl. 1486.2 8 Claims ABSTRACT OF THE DISCLOSURE A process for providing a corrosion resistant passivating film on a zinc surface, wherein the surface is treated with an aqueous solution of a chromic acid with part of the hexavalent chromium reduced to or replaced by trivalent chromium, with the pH of the solution maintained in the range of 2.8-3.8 by the addition of a pH control agent.

Zinc or zinc coated articles, for example, galvanised iron and steel, may suffer surface deterioration by corrosion through contact with the atmosphere or moisture or both. Chemical passivation treatments are widely used to inhibit or suppress such surface corrosion.

One widely accepted passivating treatment consists in treating the zinc surface with an aqueous solution of chromic acid, preferably with a part of the hexavalent chromium ions reduced to the trivalent state.

The specification belonging to US. Pat. No. 3,063,877 describes the manufacture and use of such a passivating solution wherein the hexavalent chromium is reduced by reaction with formaldehyde.

Although chromic acid based passivating solutions are widely adopted, they have been by no means completely effective in preventing corrosion under all conditions and the present invention was devised to provide a chromic acid based passivating solution and method of treating zinc or zinc-coated articles which is more effective than has been the case hitherto.

The invention is based upon the discovery that chromic acid based passivating solutions wherein some of the hexavalent chromium has been reduced to the trivalent form are rendered more effective if the pH value of the solution is maintained during the treating process within the range of from 2.8 to 3.8 by the inclusion in the solution of pH control agents chosen from the group comprising the soluble chromates, dichromates, hydroxides, or carbonates of sodium, potassium, calcium, magnesium or zinc.

If the solution is more alkaline an undesirable deposit forms on standing and if it is more acid the protection afforded becomes progressively less and an undesirable yellow colouration may develop on the treated surface depending on the level of total chromium in the solution.

The proportion of trivalent chromium to hexavalent chromium in solutions according to the invention is not critical but optimum values for corrosion resistance and lack of colouration of the passivated surface are obtained if about 20% to 30% of the chromium is in the trivalent state.

That is to say, solutions according to the invention give better protection against corrosion than do conventional reduced chromic acid solutions at the same chromium levels (conventionally from to 40% of the hexavalent chromium reduced) and at the optimum chromium level (that is with from to of the hexavalent chromium reduced) solutions of the invention not only give better protection than conventional solutions having the same surface chromium level, but also the passivating film formed is much less coloured than that produced by such conventional solutions.

A typical solution according to the invention may consist of 10 g./l. chromic acid solution in water with 25% of the chromium reduced to the trivalent form and about 5 g./l. of sodium chromate added to adjust the pH of the solution to 3.5. However, equally effective results are obtained if the chronic acid solution has from 5% to 40% of the chromium reduced to the trivalent form, provided the amount of sodium chromate added is varied to suit in each case to give a final solution with a pH in the optimum range of 2.8 to 3.8.

The solution works satisfactorily in the temperature range 20 to 70 C.; above this range a precipitate may form in the bulk of the solution which discolours the surface of the galvanised articles. However, galvanised steel to be treated may enter the solution at a higher temperature than this and may be up to about C. provided that the bulk of the solution remains within the satisfactory working range given above because the precipitate takes some time to form at the higher temperatures. The time of treatment is not critical, nor is the rate of drying of the film of solution that is carried out on the metal surface after the solution has been applied. Application conditions and solution strength are normally adjusted so that the total amount of chromium left on the surface of the article is between 1 and 4 milligrams/square foot. With less than this amount the effectiveness of the treatment falls off, and with more than this amount there is an excessive yellow or brown colour left on the surface. The solution can be applied to galvanised strip steel by any of the means presently used commercially for application of such passivating solutions.

Thus the invention extends to methods of treating zinc surfaces conforming to the above-described features.

It has been found that the solutions of the invention confer excellent protection against corrosion if formed on the surface to be passivated. Thus according to other embodiments of the methods of the invention the article to be treated is first wet with a dilute alkaline solution such as a solution of sodium hydroxide and/or sodium carbonate in water and the partly reduced chromate solution is then sprayed on to the articles surface before the alkaline film dries out.

Alternatively the steps may be reversed in sequence, and the alkaline solution sprayed on to the surface while still wet with the chromate solution.

A stringent criterion indicative of the performance of a passivating solution against corrosion of all kinds is the extent to which it inhibits the formation of a corrosion product called wet storage stain, or white rust, which is itself a serious industrial problem.

To demonstrate the efficacy of the present invention, corrugated galvanised steel sheets about three feet wide and between six and nine feet long were treated by a variety of chromic acid based passivating solutions and subsequently sprayed with distilled water and stacked under weights so that the sheets remain wet throughout the tests.

The stacked sheets may either be left undisturbed for some fixed period or opened, inspected and rewetted every week.

After such tests the galvanised steel sheets which were treated with solutions according to the invention were found to be practically free of wet storage stain after five weeks or longer, whereas sheets treated with conventional passivating solutions based on chromic acid show considerable wet storage stain after this time.

The table appearing hereinafter indicates the results of such tests more precisely. In that table the test sheets are rated on a corrosion rating scale of from to 10; 0 on the scale indicates complete coverage of the sheet by corrosion products and 10 indicates complete freedom the improvement comprising using as a constituent in said solution a sufiicient quantity of a pH control agent such that the pH value of the solution is maintained during the treating process within the range of from 2.8 to 3.8.

from corrosion. 7. A process in accordance with claim 6 wherein said It will be noticed by reference to the table that solupassivating solution is contacted with said zinc surface tions 5, 6, 7 and 8, all of which are in accordance with by forming it in situ upon said surface in accordance the present invention, produce far superior results than with the steps comprising Wetting the surface with an did the conventional solutions Nos. 1 to 4 and 9. aqueous alkaline solution and before the surface is dry Percent hexavalent Corrosion chromium rating of chromic Sodium pH of after 5 Test acid reduced, chromate working week Wet sample Solution percent addition solution Sheet appearance Stack test size 1. Aqueous chromiic acid 0 0. 5 Slight colour 1 4 6 x 3 2. Aqueous chromcc acid 0 0. 5 do 2 6. 5 3" x 3 3. Partly reduced chromic acid 1 4 6 x 4. Partly reduced chromic acid 28 2. 2 7. 5 3 x 3 5. Partly reduced chromic acid, pH adjuste 28 2. 3 9 6' x 6. Partly reduced chromic acid, pH adjusted. 28 3. 3 9 3" x 3 7. Partly reduced chromic acid, pH adjusted. 26 3. o 3 9 6 x 3 8. Partly reduced chromic acid, pH adjusted 10 3. 5 Yello 3 9 x 9. Sodium dichromate 0 4. 1 Slight colour 1 3 6 x 3 10. Untreated galvanized iron 4 0 6 x 3 1 Poor. 2 Fair. 3 Excellent.

We claim:

1. In a passivating solution for treating a zinc surface of the kind comprising an aqueous solution of chromic acid of which a part of the hexavalent chromium has been reduced to, or replaced by, trivalent chromium the improvement comprising as a constituent in the solution, a sufiicient quantity of a pH control agent, such that the pH value of the solution is maintained during the treating process within the range of from 2.8 to 3.8.

2. A solution according to claim 1 wherein from 5% to 40% of the chromium is in the trivalent state.

3. A solution according to claim 2 wherein from to of the chromium is in the trivalent state.

4. A solution according to claim 2 wherein the tritvalent chromium is introduced by dissolving a trivalent chromium oxide or hydrated compound in the chromic acid solution.

5. A solution according to claim 1 wherein said pH control agent is selected from the group consisting of soluble chromates, dichromates, hydroxides, and carbonates of sodium, potassium, calcium, magnesium, and Zinc.

6. In a treating process for inhibiting corrosion of zinc Surfaces wherein a passivating solution is brought into contact with a zinc surface and said passivating solution is of the kind comprising an aqueous solution of chromic acid of which a part of the hexavalent chromium has been reduced to or replaced by trivalent chromium,

4 Complete corrosion.

applying an aqueous solution of chromic acid, a part of the hexavalent chromium of which has been reduced to trivalent chromium, the amount of said aqueous alkaline solution applied being sufficient to maintain the pH of the solution during the treating process within the range of from 2.8 to 3.8.

8. A process in accordance with claim 6 wherein said passivating solution is contacted with said Zinc surface by forming it in situ upon said surface in accordance with the steps comprising wetting the surface with an aqueous solution of chromic acid, a part of the hexavalent chromium of which has been reduced to trivalent chromium, and before the surface dries treating it with an aqueous alkaline solution in an amount sufficient to control the pH of the solution during the treating process within the range of from 2.8 to 3.8.

References Cited UNITED STATES PATENTS 2,762,731 9/1956 Heller et a1. 1486.2 3,154,438 10/1964 Keller et a1. 148-6.2X 3,282,744 11/1966 Goldsmith 1486.2 3,318,716 5/1967 Schuster et al. 1486.2 3,391,032 7/1968 Hansen et al. 148--6.16 3,404,045 10/ 1968 Coulman 148-6.2X

RALPH'S. KENDALL, Primary Examiner 

